The present invention relates generally to ultrasonic transducers, and, more particularly, to an efficient micro-machined ultrasonic transducer (MUT) array.
Ultrasonic transducers have been available for quite some time and are particularly useful for non-invasive medical diagnostic imaging. Ultrasonic transducers are typically formed of either piezoelectric elements or of micro-machined ultrasonic transducer (MUT) elements. The piezoelectric elements typically are made of a piezoelectric ceramic such as lead-zirconate-titanate (PZT), with a plurality of elements being arranged to form a transducer. A MUT is formed using known semiconductor manufacturing techniques resulting in a capacitive ultrasonic transducer cell that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate. By applying contact material, in the form of electrodes, to the membrane, or a portion of the membrane, and to the base of the cavity in the silicon substrate, and then applying appropriate voltage signals to the electrodes, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, when electrically biased, the membrane of the MUT may be used to receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the electrically biased membrane, which then generates a receive signal.
The ultrasonic transducer elements may be combined with control circuitry forming a transducer assembly, which is then further assembled into a housing possibly including additional control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include various acoustic matching layers, backing layers, and de-matching layers may then be used to send and receive ultrasonic signals through body tissue.
In the past, MUT arrays were typically designed where each MUT element was a transceiver. In such an arrangement, each MUT element both produces a transmit pulse and receives acoustic energy. Unfortunately, the characteristics of a MUT element that make it a good transmitter of acoustic energy are not the same characteristics that make it a good receiver of acoustic energy. For example, during a transmit pulse, it is desirable for the MUT to provide a large power output. To accomplish this, a large membrane deflection, a large gap, high membrane stiffness, and high bias voltage are desired to produce the high pressure wave desired on transmit. In such a MUT, the cavity depth should be at least three times deeper than the static deflection of the membrane. Membrane deflection larger than approximately ⅓ of the cavity depth result in the collapse of the membrane against the cavity floor. The gap is defined as the distance between the membrane and the bottom of the cavity. A large gap results in a small capacitance and large imaginary impedance. Ideally a bias voltage is applied to deflect the membrane and reduce the gap to the minimum uncollapsed size.
Conversely, for a MUT to be a sensitive acoustic receiver, a small membrane deflection, a small gap, low membrane stiffness, and high bias voltage produce a sensitive acoustic receiver element. The small gap reduces the imaginary impedance and the soft membrane deflects easily when exposed to acoustic energy reflected from a target resulting in a high signal-to-noise ratio (SNR).
Therefore, it would be desirable to have a MUT array in which the individual MUT elements can be independently optimized for transmit and receive functionality.
An ultrasonic transducer array comprising individual transmit MUT elements and receive MUT elements where the transmit MUT elements and the receive MUT elements are distributed in two dimensions over the transducer array is disclosed. By using different MUT elements for transmit and receive, each MUT element can be independently optimized for either transmit operation or receive operation. Furthermore, by independently optimizing the MUT elements for either transmit or receive operation, the same bias voltage can be applied to the MUT elements, thereby simplifying the bias circuitry associated with the MUT transducer array. Alternatively, because the MUT elements are independently optimized for transmit and receive, different bias voltages can be applied to the transmit and receive elements, thus providing further optimization of the elements.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.